In recent years, the so-called RF plasma CVD process has been frequently used for the production of semiconductor devices. In the RF plasma CVD process, a high frequency of 13.56 MHz is generally used in view of the Wireless Telegraphy Act. The RF plasma CVD process has advantages in that the discharge conditions can be relatively easily controlled and the quality of a film obtained is excellent, but it has drawbacks in that the utilization efficiency of a gas is not sufficient and the deposition rate of a film is relatively small. In order to solve these drawbacks in the RF plasma CVD process, there has been proposed a microwave plasma CVD process using the so-called microwave with a frequency of 2.45 GHz. The microwave CVD process has advantages in that the utilization efficiency of a gas is sufficient and the deposition rate of a film can be significantly increased, but has drawbacks in that the plasma density upon film formation is significantly high and thereby a raw gas is abruptly decomposed and the film deposition is made at a high deposition rate so that it is very difficult to stably form a dense film.
In view of the foregoing, studies have been recently made for a plasma CVD process using a very-high-frequency of the so-called VHF range, having a frequency of 30 MHz to 150 MHz which is greater than the 13.56 MHz. For example, in Plasma Chemistry and Plasma Proceeding, Vol. 7, No. 3, pp/267-273 (1987) (hereinafter, referred to as Document 1), there is described a film-forming technique using a glow discharge decomposition apparatus of a capacitance coupling type wherein a raw gas (silane gas) is decomposed by a very-high-frequency energy of 25 MHz to 150 MHz to form an amorphous silicon (a-Si) film. Specifically, Document 1 describes that a-Si films are formed with the frequency changed in the range of 20 MHz to 150 MHz, and in the case of using a frequency of 70 MHz, the film deposition rate becomes the maximum value, 21 .ANG./sec, which is about 5 to 8 times that in the RF plasma CVD process; and that the defect density, optical band gap and conductivity of the resulting a-Si film are not affected by the exciting frequency so much. Document 1, however, only discloses the film formation on a laboratory scale, and it does not describe whether or not the above-described effect are applied to the formation of a large area film at all. In addition, Document 1 merely suggests a possibility that the use of higher frequencies (13.56 MHz to 200 MHz) leads to the interesting perspective for fast processing of an inexpensive and large area a-Si:H film device required for a thickness of several .mu.m. In this regard, as is apparent from the experimental results (described later) by the present inventors, the use of a very-high-frequency energy in the so-called VHF range can achieve the high decomposition efficiency of a gas and high deposition rate, but it cannot bring about the formation of a practically usable large area deposited film. In addition, Japanese Patent Laid-open No. 64466/1991 (hereinafter, referred to as Document 2) discloses a technique of forming an amorphous silicon based semiconductor film on a cylindrical substrate by use of a very-high-frequency energy with a frequency of 20 MHz or more (preferably, 30 MHz to 50 MHz). Specifically, a raw gas is introduced into a reaction chamber; the interior of the reaction chamber is kept at a gas pressure of 10.sup.-4 to 0.2 Torr; and a very-high-frequency energy in an amount corresponding to 0.1 to 10 W/sccm in terms of a ratio to the flow rate of the raw gas is introduced into the reaction chamber to cause glow discharge thereby forming an amorphous silicon based semiconductor film. According to the description of Document 1, a film deposition rate of 10 .mu.m/hr or more can be obtained and the unevenness of the thickness in the deposited film can be suppressed to be 20% or less.
The technique of Document 2, however, fails to obtain the above film deposition rate by use of a very-high-frequency with a frequency beyond the above-described frequency range. Namely, as an experimental result (described later) by the present inventors that the technique described in Document 2 is practiced using a high frequency power supply with a frequency of 40 MHz or more, it is revealed that for the frequency of 60 MHz or more, there occurs the unevenness of the thickness in a deposited film in either of the axial and circumferential directions of a cylindrical substrate, and a film having a good quality cannot be obtained at a high deposition rate.